1. Technical Field of the Invention
This invention relates to compositions useful in the preparation of semiconductive conductor shields in power cables and to semiconductive conductor shields and power cables utilizing the composition.
2. Description of the Related Art
A typical insulated electric power cable generally comprises a conductor in a cable core that is surrounded by several layers of polymeric materials including an inner semiconducting shield layer (conductor or strand shield), an insulating layer, an outer semiconducting shield layer (insulation shield), a metallic wire or tape shield used as the ground phase, and a protective jacket. Additional layers within this construction such as moisture impervious materials, are often incorporated. The invention pertains to the inner semiconducting shield layer, i.e., the conductor shield.
Semiconductive shields have been used in power cables as shields for the cable conductor and insulation for many years. The conductor shield is typically extruded over the cable conductor to provide a layer of intermediate conductivity between the conductor and cable insulation in the power cable. Conventional compositions for these conductor shields include a base polymer as the predominant component of the composition compounded with, carbon black to provide conductivity for the composition and various additives.
The primary purpose of the semiconducting conductor shield between the conductor and insulation in an electrical power cable is to ensure the long term viability of the primary insulation. There is always a need for improved semiconductive conductor shield compositions that balance cost and performance.
Examples of polymer compositions used as shields in power cables are found in the disclosures of U.S. Pat. Nos. 4,612,139 and 4,305,849 to Kawasaki et al., U.S. Pat. No. 4,857,232 to Burns, Jr., U.S. Pat. No. 3,849,333 to Lloyd et al., U.S. Pat. No. 5,889,117 to Flenniken, and U.S. Pat. No. 6,086,792 to Reid et al., the disclosures of which are hereby incorporated by reference.
U.S. Pat. No. 6,491,849 to Easter, the disclosure of which is hereby incorporated by reference, describes a semiconducting composition containing EVA wax, but not containing polyethylene glycol.
It would be desirable to have a conductor shield material with improved performance that does not require the use of expensive additives, complex polymer formulations, or specially prepared carbon black, as performance must always be balanced with cost in the manufacture of electric cable.